1. Field of the Invention
The present invention relates generally to an apparatus which uses a vaporized processing fluid to heat a workpiece to a preselected temperature, and particularly to a vapor phase soldering system which heats a workpiece such as a circuit board with a vaporized processing fluid to reflow solder carried by the workpiece.
2. Description of the Prior Art
Vapor phase soldering systems are known in the prior art. Typically, a processing, treatment or working fluid having a heavier-than-air vapor phase is heated to its boiling point in a chamber of a vessel. The chamber is open to the atmosphere through openings in the wall of the vessel to facilitate movement of a workpiece, such as a circuit board, therethrough. Solder on the workpiece is heated to above its melting point by the vapor phase of the working fluid to melt or reflow the solder on the circuit board. When the circuit board is conveyed out of the chamber, the solder solidifies, permanently connecting the electrical elements to the circuit board. U.S. Pat. No. 4,389,797 granted June 28, 1983 to Spigarelli et al.; U.S. Pat. No. Re. 30,399 granted Sept. 9, 1980 to Ammann et al.; U.S. Pat. No. 4,394,802 granted July 26, 1983 to Spigarelli; U.S. Pat. No. 4,055,217 granted Oct. 25, 1977 to Chu et al., and U.S. Pat. No. 4,077,467 granted Mar. 7, 1978 to Spigarelli are representative of this type of vapor phase processing system.
One disadvantage of these prior art systems is that the processing vapor escapes from the vessel chamber through the openings in the walls of the vessel through which the circuit board is conveyed. Release of the processing vapor into the atmosphere is both expensive, due to the high cost of most processing vapors, and poses a health risk.
To minimize vapor loss, the vapor processing system disclosed in U.S. Pat. No. 4,389,797 to Spigarelli et al. utilizes elongated first and second conduits in communication with the chamber of the vessel, through which the workpiece enters and exits the chamber. Cooling coils are provided along the length of the conduit to condense and recover the processing fluid before it escapes into the atmosphere. However, cooling the vapor within the first and second conduits will tend to cause the vapor to condense on the workpiece itself. Thus, the processing liquid is still carried out of the chamber and the workpiece will have to be cleaned after processing to remove the processing fluid.
U.S. Pat. No. Re. 30,399 to Ammann et al. shows apparatus which utilizes substantially vertically oriented inlet and outlet openings to the vapor processing chamber, through which the workpiece enters and exits the chamber. Cooling coils are disposed within the inlet and outlet openings to prevent the vapor from being released into the atmosphere. Again, the vapor can condense on and be carried away by the workpiece. Further, the circuit board or other workpiece is carried into and out of the chamber of the vessel at an angle. This movement of the circuit board out of a horizontal orientation during melting or reflow adversely affects the finished product.
The apparatus shown in U.S. Pat. No. 4,394,802 to Spigarelli retains the processing vapor within the chamber of a vessel by creating a counter-convection flow which opposes an outward convection flow of the processing vapor from an opening in the vessel. This apparatus can only be used for treating workpieces which can be lowered vertically into the vessel and hoisted vertically from it.
U.S. Pat. No. 4,055,217 to Chu et al. shows a system which utilizes a blanket of secondary vapor disposed intermediate a hot primary vapor and the atmosphere. The secondary vapor acts on the primary vapor to condense it, preventing loss of the primary vapor to the atmosphere. This system, like Spigarelli '802, can only be used for vertically moving batches.
Similarly, U.S. Pat. No. 4,077,467 to Spigarelli discloses a method and apparatus for drawing a workpiece through a plurality of substantially vertically aligned confined regions of saturated vapors. The workpiece is initially heated in a primary vapor region and is sequentially raised through upper vapor regions. Vapors escaping from lower regions into the upper regions are condensed in the upper regions and are returned to a lower region fluid reservoir. Horizontal batch or continuous flow-through operation is not possible.